What’s New

TL;DR
When do I swap to T21?
You probably should already be running it, T21 can see benefit in as little as 10-20 ilvls per piece above T20 depending on how well you are able to weave with it.
How do I play to T21's bonuses?
Binding Heal has really good synergy with it. Binding Heal for low damage, Binding Heal/Prayer of Healing weaving for medium/high raid damage. (Remember to use Prayer of Mending and Holy Words too while weaving!)
Should I drop T21 for higher ilvl off pieces?
It is really unlikely it would be worth it, stick with T21 unless you have some amazing TFs.
Gilded Seraph's Initial Thoughts
Upon release of the Gilded Seraph Raiment (T21) set bonuses, the initial feeling was that it was an underwhelming set. This was a reasonable

It's that time again!
The next WoW expansion Battle for Azeroth can now be previewed in the newly released Beta, here we'll try to cover most of the updates to Holy Priest over the duration of the Beta/Alpha/PTR.
Beta, Alpha and PTR changes a lot. Nothing is final, data-mining has errors... don't panic! There is a long way to go yet.
You'll see many things that are data-mined that are incorrect, lots of speculation and an incredible amount of FUD. Just be patient and keep an eye on how things progress as nothing is final until it's released!
BFA Beta Build 25902 - 26th Jan 2018
The first BFA beta build is out with a few minor changes for Holy Priest. You can view these changes on both the Wowhead build details or the MMOC spell updates thread.
PLEASE NOTE: Some of these changes...

With the opening of Antorus coming soon, and patch notes released, this writeup aims to provide a quick synopsis of the unique gear that will help us do our job, and keep the raid alive. As with every other raid opening in Legion, best in slot (BiS) list is an outdated term with the current design of the titanforge (TF) system. Trinkets are kind of the exception to this, but after a certain point, the higher ilvl of a TF may win out.
Tier 21
The design of our tier 21 set bonuses aim to give us more efficient filler spells when we alternate casts or "spellweave" them together.
Item - Priest T21 Holy 2P Bonus - Prayer of Healing increases the healing done by your next Flash Heal/Binding Heal/Heal by 60%.
Item - Priest T21 Holy 4P Bonus - Flash Heal/Binding Heal/Heal increases the hea...

Enduring Renewal is a talent that's had a bit of a hard time this expansion. Prior to 7.1.5, it suffered from being on the same talent tier as Trail of Light, which back then allowed Blessing of T'uure to have two chances to proc when casting Flash Heal. This meant that ToL was such a strong talent that ER couldn't even come close to competing. When 7.1.5 was released, the extra BoT proc chance was effectively removed from ToL, but changes were made to PoH which pushed us towards filling with PoH when possible. This meant that Enlightenment was generally a better choice except on fights where you needed strong single target throughput, where it was a good choice to switch to ToL to make spot healing checks. In general, prior to 7.2.5, Enduring Renewal pretty much only saw play as part of t...

So Why Binding Heal Now?
Binding Heal is an iconic and longstanding healing priest spell that has seen many iterations. This expansion has seen a few iterations:
7.0.1 - 3% Base Mana - Heals Caster, Target and a Friendly for 200% SP
7.0.1 - 3% Base Mana - Heals Caster, Target and a Friendly for 250% SP
7.0.3 - 3% Base Mana - Heals Caster, Target and a Friendly for 300% SP
7.1.0 - 2.5% Base Mana - Heals Caster, Target and a Friendly for 300% SP
7.2.5 - 1.7% Base Mana - Heals whoever it feels like for 300% SP
7.2.5 - 1.7% Base Mana - Heals Caster, Target and a Friendly for 300% SP
During 7.0 it was competing against SoL, which was easily the most desirable talent due to its interaction with ToL. Then in 7.1, buffs to Piety and PoH throughput (coupled with ToL to no lo

So now that it's become just a bit more relevant due to Binding Heal's newfound strength, I'd like to talk about a bug that has to do with the new gold trait Holy Priests recieved in 7.2, Cosmic Ripple. This bug has been around since CR was introduced, and still hasn't been fixed to this day.
Update 9/7/17 - Cosmic Ripple Fixed
Since posting this, Cosmic Ripple has now been fixed and no longer procs multiple times as discussed in this post. 3 and a bit months later and Cosmic Ripple is not still broken...
What is Cosmic Ripple?
Cosmic Ripple was Holy's fourth golden artifact trait introduced in 7.2. It's a pretty strong increase, doing 3-4% of your healing while also triggering Echo of Light. It's worth nothing that the original implementation of the trait was bugged badly. Upon Cosmic

Binding Heal has had a hard time this expansion, starting with 7.0 where Surge of Light was really valuable, and continuing into 7.1.5 when we moved on to Piety as our primary raiding talent choice on the T75 tier. With the recent tuning changes in 7.2.5 people are trying Binding Heal once more and the results are... interesting to say the least.
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Update 28/6/17
So there was a hotfix overnight which slightly changed how the trinket works, bringing it more in line with other ToS trinkets. Depending on overheal values it's still on top, but it's no longer on top by as much. Here's what changed:
Sea Star of the Depthmother’s Ocean's Embrace now has an 8 second internal cooldown. The proc chance has been reduced slightly for Holy Paladins and Holy Priests.
Sea Star now has a 40% chance to proc
The buff now has an ICD equal to it's duration, preventing PoH being able to refresh it
The trinket spreadsheet has been updated with this latest information and it's showing it's still a really strong trinket but is no longer eclipsing the healing from the other top ToS trinkets.
Update 27/6/17
Over the weekend some of the Blizzard